Television scanner cathode



Dec. 6, 1938. w. HEIMANN' 2,139,018

TELEVIS ION SCANNER CATHODE Filed Sept. 21, 1957 JEHEITIWTY LENGTH OF CATHODE WIDTH OF CATHODE IN VEN TOR WAL TE I? HE IMA N/V.

A TTORNEYS.

Patented Dec. 6, 1938 g 5 UNITED STATES PATENT OFFICE 2,139,018 7 TELEVISION SCANNER CATHODE Walter Heimann, Berlin-Tempelhcf, Germany, assignor to the firm Fernseh Aktiengesellschaft, Zehlendorf, near Berlin, Germany Application September 21, 1937, Serial No. 164,942 In Germany September 10, 1936 4 Claims. (Cl. 250-167) This invention relates to television scanners, drawing, wherein Fig. 1 is a schematic sectional and particularly to Scanners of the type publicized view of a cathode ray tube embodying my inby Vladimir K. Zworykin under the name Iconovention. Figure 2 is a diagram of a conventional 1 rectangular mosaic photocathode, with coordi- The efiiciency of the known cathode ray beam nate axes indicated thereon. 5

scanner or Iconoscope of Zworykin, is con- Figure 3 comprises graphs showing the variasiderably increased by increasing the beam curtions of photosensitivity and secondary electron rent, which causes an increase of contrast in the emissivity along said axes in accordance with this image. When increasing the beam current, howinvention.

ever, a disturbing eiTect in the picture appears in Figures 4 and 5 indicate the variations of these 10 the form of non-uniform and unsymmetrical dark quantities along the vertical and horizontal axes portions which extend over wide portions of the in accordance with a modification of the inarea of the image. This interference is an intervention. fering impulse superimposed upon the picture In Fig. 1 there has been disclosed in somewhat 1.; signal, which interfering impulse consists of a schematic form a cathode ray device of the type series of frequencies, particularly the line and previously referred to, such a tube comprising an frame frequency, and is mainly caused by space envelope 1 which houses the necessary electrodes charge in front of the photocathode. The space including a source of electrons 4, a. grid or concharge is presumably produced by strong secondtrol electrode 5, an anode 6, deflecting plates 1 ary emission from the photocathode, It is parand 8, and a photocathode l I, all arranged in 20 ticularly noticeable that the interference always conventional manner well known in the art. The

migrates from the periphery of the cathode topresent invention relates particularly to improve- Wards the center of the plate when increasing ments in the photocathode electrode which is the beam current. Certain effects of the scan sometimes referred to as a mosaic cathode by ning beam, particularly the phenomena caused reason of the fact that its surface is composed of by the return trace and by non-linearity, furtherminute globules photosensitized so as to render more often cause the interference to migrate from them responsive to the effects of light. one side over the entire area. of the cathode. Figure 2 shows a mosaic cathode H of rectan- It has been proposed to reduce this interfering gular shape. The cathode comprises a photo impulse by exterior means, such as by superimmosaic which may be produced in any desirable posing a corresponding compensating voltage. manner, as has become well known in the art. In practice, however, this measure is very difii- In the usual photosensitive layers, the basic silver cult and necessitates extensive equipment. Furlayer, comprising droplets of silver condensed on thermore, it has been proposed to reduce the an insulating plate, is oxidized, and the photointerfering impulse by a favorable arrangement sensitivity as well as the secondary emission ratio of the accelerating electrodes. depends on the degree of oxidation. Experience According to this invention, a further and conhas shown that the photosensitivity variesin siderable reduction or elimination of the inter-' opposite sense to the secondary emission ratio, fering impulse is accomplished by producing 2. whereby the secondary emission ratio increases photocathode, which is non-uniform in respect with increasing degree of oxidation while the 40 to its light sensitivity and, if necessary, asymphotosensitivity decreases. If the periphery of metrical. It is particularly preferable to allow the plate has been subjected toa stronger degree the light sensitivity to increase towards the peof oxidation, the interfering impulses will be riphery. It may also be of advantage to vary especially great. Thus, according to the invenover the photocathode, according to a. predetertion, the center of the photocathode is subjected mined law, the ratio of effective photosensitive to stronger oxidation than the periphery and in surface to the area of space between the indisuch a ratio that a reduction or elimination of the vidual photosensitive particles. This may be interfering impulses is gained. accomplished by either enlarging or diminishing Figure 3 shows the photosensitivity along the so the distances for constant size of the particles, line 33 of the mosaic plate. Normally, a

or to make the distance between particles constraight horizontal line would be obtained. Acstant and to vary the average size of the particles cording to the invention, however, the photoat different portions of the cathode. sensitivity follows the line l2 and the secondary In the following, the object of the invention emission ratio the line I4. The variation of these may be explained in detail with the aid of the values along the line 4-4 is shown in Figure 4, 55

where the photosensitivity follows the line l6 and the secondary emission ratio the line l1.

Such a distribution of the sensitivity is, for instance, accomplished by locally controlling the oxidizing process. Oxidation is accomplished by passing a current from an auxiliary electrode to the cathode in an atmosphere of oxygen at low pressure, causing a glow discharge, and the degree of oxidation is a function of the intensity of the glow and the time during which it is applied. If, for instance, the oxidation is made in a glow discharge which is produced between an auxiliary electrode disposed on one side of the mica plate and an anode,itis possible to oxidize the center more strongly than the periphery by suitable choice of thevapor pressure or by choosing a suitable shape for the auxiliary electrode or both. Whereas an approximately homogeneous oxidation takes place at a vapor pressure of 0.01 millimeter Hg, the oxidation at vapor pressure of the order of several thousandths millimeter Hg is influenced in such a manner that the desired distribution of sensitivity is obtained.

It is possible to let the positive ions impact substantially only the center portion of the plate. Thus, with a point auxiliary electrode apposed to the center of the plate, and the plate mounted temporarily or permanently in an oxygen filled envelope on the pump, the discharge glow at the higher pressure mentioned will be sensibly uni form over the whole cathode area, but as the pressure is further reduced the glow will concentrate at the center of the cathode, producing the heaviest oxidation at this spot, and the distribution of sensitivity shown in Figures 3 and 4 will be closely approximated.

To produce other patterns the auxiliary electrode may be disposed at the periphery of the mosaic cathode, or only at different parts of the periphery. By enlarging or decreasing the distance, the oxidation process can be influenced considerably.

The distribution of sensitivity shown in Figures 3 and 4 only serves as an example. A somewhat difierent distribution is shown in Figure 5, whereby the photosensitivity is to remain constant along the line 44. The distribution of sensitivity over the plate is, therefore, different for the two coordinates. Such a distribution may be produced by means of a linear or knife-edged electrode apposed to the center of the plate. It

may also be asymmetrical and increase only on one side. Also, the actual difierence in sensitivity between portions of highest and lowest sensitivity may vary within wide limits. The ratio of about X :2X shown in the drawing, however, is sufiicient to cause a reduction of the interfering impulse. The desired distribution of sensitivity is applicable to cathode ray scanners with one-sided as Well as double-sided mosaic plates.

While control of sensitivity is preferably obtained by the control of oxidation, as above described, it may also be accomplished in considerable degree by regulating the deposit of silver on the insulating plate N. This is sometimes done by masking ofi parts of the plate, but it may also be accomplished by evaporating the silver onto the plate from a point sumciently near so that there is an appreciable difference in distance from that point to the different parts of the plate, the heavier deposit occurring at the nearer points, the size of droplets of condensed silver, or their density of distribution, or both, being increased by increased proximity to the point of evaporation. These latter factors can also be varied in some degree, as is well known, by the gas pressure and the speed at which evaporaton takes place.

The final sensitization of the cathode, by admitting caesium or other photosensitive material, is carried on in accordance with usual practice.

I claim:

1. The method of forming a photosensitive cathode which comprises depositing a silver mosaic upon an insulating plate, oxidizing said silver mosaic in a glow discharge in an atmosphere of rarefied oxygen at a pressure of the order of thousandths of a millimeter of mercury and then sensitizing the surface of said plate with an alkali metal.

2. A photocathode having a coating of photosensitive material varying in sensitivity in accordance with a predetermined pattern.

3. A photocathode having a coating of photosensitive material, the photosensitivity of which increases from the center of the cathode toward its periphery.

4. A photocathode having a coating of photosensitive material varying uniformly in photosensitivity along one axis from the center of said cathode to its periphery.

WALTER HEIMANN. 

